HR: 0830hAN: T11D-0429TI: The West Napa Fault as defined by gravity and magnetic data, northern CaliforniaAU: * Langenheim, V EEM: zulanger@usgs.govAF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United StatesAU: Graymer, R WEM: rgraymer@usgs.govAF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United StatesAU: Jachens, R CEM: jachens@usgs.govAF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United StatesAU: McPhee, D KEM: dmcphee@usgs.govAF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United StatesAU: Schmidt, K MEM: kschmidt@usgs.govAF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United StatesAB: The north-northwest-striking West Napa Fault is mapped along the western margin of Napa Valley, California. The epicenter of the 2000 M$_{L}$5.2 Yountville earthquake occurred 5 km west of the surface trace of the West Napa Fault as defined by Helley and Herd (1977), and thus the source of the earthquake was assigned to an unmapped fault. Here we argue based on recently acquired geophysical and geologic data that the earthquake may have occurred on a strand of the West Napa Fault. A linear aeromagnetic anomaly along strike with the Holocene West Napa Fault extends northwest 30 km from just north of the Napa County Airport to the latitude of the town of Rutherford. North of Rutherford, another linear aeromagnetic anomaly can be traced 20 km north to Calistoga. The source of the anomalies resides within the pre-Cenozoic basement rocks, most likely unexposed ophiolitic basement rocks of the Great Valley sequence. Both of the aeromagnetic anomalies occur near the base of a linear east-facing gravity gradient. The gravity gradient is caused by the juxtaposition of Great Valley and Franciscan rocks to the southwest with less dense Cenozoic Sonoma volcanics and alluvium to the northeast, although this relationship is completely buried by Sonoma volcanics at the surface north of the town of St. Helena. The correlation of the potential-field anomalies suggests that a steeply west-dipping reverse fault bounds the western margin of the Napa Valley basin. The alignment of the reverse fault with the Holocene mapped West Napa Fault suggests that they are related. The focal mechanism of the Yountville earthquake, which occurred at a depth of about 10 km, indicates slip occurred on a steeply southwest-dipping, northwest-striking fault plane. Projection of this fault plane to the surface coincides closely with the location of the geophysically defined fault bounding the western margin of the Napa Valley basin and the surface trace of the West Napa Fault as mapped by Fox (1983) and Graymer and others (2003). Although the focal mechanism indicates nearly pure right-lateral slip, aftershocks of the event include both right-lateral and reverse mechanisms. Despite the relatively small magnitude of the Yountville earthquake, it probably occurred on a fault capable of much larger earthquakes. Given the length of the geophysically defined West Napa Fault, it may be capable of producing a {\bf M} 6.8-7.1 earthquake. An unusual characteristic of the Yountville earthquake was more extensive damage in the city of Napa than in communities more proximal to the epicenter. A preliminary inversion of the gravity data indicates that the Cenozoic basin fill is as much as 2 km thick beneath the town of Napa and substantially thinner beneath Yountville. The variation in thickness of the basin fill may be one of the factors that contributed to unusually strong ground accelerations recorded at Napa and the lack of damage to older buildings at Yountville during the 2000 earthquake. DE: 1219 Local gravity anomalies and crustal structureDE: 7223 Seismic hazard assessment and predictionDE: 7230 Seismicity and seismotectonicsDE: 8015 Local crustal structureDE: 8107 Continental neotectonicsSC: Tectonophysics [T]MN: 2003 Fall Meeting